Hitherto, color recording techniques for facsimille printers, copying machines, or television images have been desired and color recording techniques by electrothermal ink jet printings, thermal transfer recording, etc., have been investigated.
In these techniques, a thermal transfer recording system is considered to be advantageous as compared to other systems since in this system, the apparatus can be easily maintained and operated and also the apparatus supplies are inexpensive.
As a thermal transfer recording system, there are a melting system wherein a transfer sheet having heat-meltable ink layer formed on a base film is heated by a heating means such as thermal head, infrared rays, etc., to melt the ink and molten ink is transferred onto a recording material to perform recording and a sublimation system wherein a transfer sheet having an ink layer containing sublimable dyes formed on a base film is heated by a heating means to sublime the dyes and the sublimed dyes are transferred onto a recording material to perform recording. In the sublimation system, the sublimed and transferred amount of dyes can be controlled by changing the energy for heating the dyes to facilitate gradation recording and hence the sublimation system is considered to be particularly advantageous for full color recording.
In thermal transfer recording by the sublimation system, sublimable dyes which are used for a transfer sheet are very important since they give large influences on the thermal recording speed, the image quality, storage stability, etc., of the record and are required to meet the following conditions:
(1) The dye easily sublimes by the working condition of an ordinary heating means such as a thermal recording head, etc. PA1 (2) The dye does not cause thermal decomposition under the aforesaid working condition before sublimation. PA1 (3) The dye has a preferred hue for color reproduction. PA1 (4) The dye has a large molecular extinction coefficient. PA1 (5) The dye is stable to heat, light, moisture, chemicals, etc. PA1 (6) The dye can be easily synthesized. PA1 (7) The dye is excellent in ink-making aptitude.
Furthermore, for obtaining black recording by thermal transfer recording of sublimation system, transfer recording is performed using a transfer sheet having a black color material layer of a mixture of yellow, magenta, and cyan colors but conventional black transfer sheets are yet insufficient in performance and hence the development of a black transfer sheet having satisfactory performance has been desired.